1. Field of the Invention
The present invention relates to a spread illuminating apparatus of side light type for use as a lighting means for a liquid crystal display device, and particularly to a spread illuminating apparatus suitable for use as a lighting means for a color liquid crystal display device for a compact mobile terminal, such as a mobile telephone.
2. Description of the Related Art
Currently, a spread illuminating apparatus of side light type in which a white light emitting diode (LED) as a primary light source is disposed at a side face (light inlet surface) of a light conductor plate is extensively used as a lighting means for a liquid crystal display (LCD) device with a relatively small display area for a compact mobile terminal, such as a mobile telephone. Such the white LED generally includes in combination a semiconductor light emitting element to emit a blue light and a fluorescent substance to absorb a blue light and emit a yellow light, whereby a spectrum distribution to produce a white light is developed (the white LED is referred to as “B+Y type LED”).
Recently, a mobile terminal, such as a mobile telephone, increases its application and now functions as a video terminal to watch television programs, and a color LDC device for such the terminal is desired to present a high picture quality. The light emitted by the aforementioned B+Y type LED, however, is poor in a red spectral component and therefore is not perfectly suited for a video terminal to show television movies or the like which requires high color rending properties.
Under the circumstances described above, when high color rendering properties are required, an LED is used which includes in combination a semiconductor light emitting element to emit a blue light, a fluorescent substance to absorb a blue light and emit a red light, and another fluorescent substance to absorb a blue light and emit a light of a color other than red (for example, a yellow or green color), thereby realizing a spectrum distribution to produce a white light with enhanced color rendering properties (such the white LED is referred to as “B+RX type LED”, where “X” may be, for example, “Y” for yellow or “G” for green) as disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-244021.
The B+RX type LED, however, is inevitably deteriorated in visibility and conversion efficiency due to the increase of the red spectral component or achieves a smaller light beam than the B+Y type LED due to the increase of the fluorescent substance to absorb the blue light emitted from the semiconductor light emitting element.
The color rendering properties have been enhanced, for example, by employing a combination of a plurality of LEDs which incorporate respective semiconductor light emitting elements adapted to emit lights of respective different colors (typically, red, green and blue colors) as disclosed in, for example, Japanese Patent Application Laid-Open No. 2001-319514, or by employing an LED which includes such the respective semiconductor light emitting elements in one package (LED package) as disclosed in, for example, Japanese Patent Application Laid-Open No. 2005-183124 (the plurality of LEDs and the LED package are referred collectively as “RGB type LED” as appropriate). Such the RGB type LED emits a green light having a low luminance efficiency and therefore, like the B+RX type LED, achieves a smaller light beam than the B+Y type LED. And, furthermore, a power supply circuit is needed for each of the plurality of semiconductor light emitting elements, thus inviting a dimensional increase of the device.
The above-described LEDs of different types are adapted to emit white lights based on different light emitting principles and accordingly have different light emitting properties. Under such the circumstances, a spread illuminating apparatus is disclosed which incorporates a light source having the different types of white LEDs with the different light emitting principles in order to properly balance the luminance efficiency and the color rendering properties (refer to, for example, Japanese Patent Application Laid-Open No. 2005-203326).
Since a color LCD device for a mobile terminal, such as a mobile telephone, has a variety of usages in various environments, the properties of the light source have to be optimally controlled according to the usages and environments. For example, for reading characters in electronic mails, the color rendering properties do not necessarily have to be high, and the luminance efficiency receives priority. On the other hand, for watching pictures in the dark environments, the color rendering properties are important while a high brightness is not necessarily required. And, for use in the light environments, a high brightness is desired.
In the spread illuminating apparatus disclosed in, for example, the aforementioned Japanese Patent Application Laid-Open No. 2005-203326 where an illuminating light having specific properties is produced by simply turning on the different types of LEDs on a selective basis, it is difficult to achieve a decent brightness distribution across all lighting modes: specifically, the LEDs are all turned on at the same time, the LEDs are selectively turned on and off, and the like.